Nickel base superalloys are widely used in applications where high stresses must be endured at elevated temperatures. One such application is in the gas turbine engine industry, especially for turbine blades and vanes. Demands for improved efficiency and performance have resulted in the operation of gas turbine engines at increasingly elevated temperatures placing extreme demands on the superalloy articles used therein.
Conventionally prepared metallic materials are comprised of a plurality of grains which are separated by grain boundaries. The grain boundaries are weak at elevated temperatures, much weaker than the material within the grains. Such grain boundaries are prominent sites for crack initiation at elevated temperatures. Accordingly, elimination of high angle grain boundaries reduces the propensity for crack formation which in turn increases component life at elevated temperatures.
One approach which has been successfully employed is to fabricate the metallic materials in the form of single crystals. Through specific casting techniques nickel base superalloys can be produced in single crystal form, that is, in the form of articles which have no internal grain boundaries. U.S. Pat. No. 3,260,505 describes nickel base single crystal superalloy articles. U.S. Pat. No. 4,116,723 describes a heat treatment method applicable to a class of nickel single crystal articles. U.S. Pat. No. 4,209,348 describes a more specific class of single crystal articles and describes a method for heat treating such articles to improve their elevated temperature mechanical properties. The nominal composition disclosed in the '348 patent, in weight percent, is 10% chromium, 5% aluminum, 1.5% titanium, 4% tungsten, 12% tantalum, 5% cobalt, balance essentially nickel. Another single crystal superalloy composition is that which is described in U.S. Pat. No. 4,402,772 having a nominal composition, in weight percent, of 6% tantalum, 9% chromium, 5% cobalt, 1% titanium, 1% molybdenum, 7% tungsten, 5.5% aluminum, 0.15% hafnium, balance essentially nickel. Yet another composition is that described in U.S. Pat. No. 4,222,794 having a nominal composition, in weight percent, of 5.2% chromium, 5.4% aluminum, 1.1 % titanium, 2% molybdenum, 4.9% tungsten, 6.4% tantalum, 3% rhenium, 0.4% vanadium, balance essentially nickel. Still another composition is that which is described in U.S. Pat. No. 4,719,080 having an exemplary and preferred composition, in weight percent of, 5% chromium, 2% molybdenum, 6% tungsten, 3% rhenium, 9% tantalum, 5.6% aluminum, 10% cobalt, 0.1% hafnium, balance essentially nickel.